The purpose of the proposed research is to study various aspects of the metabolic relationship between vitamin B12 and folic acid. The specific objectives include: 1) The study of the metabolic relationship between folic acid, vitamin B12 and methionine with special reference to the quantitative aspects of the methyl trap theory. 2) The study of the effect of thyroxine on folic acid metabolism. 3) The isolation and identification of "x-methyl folic acid" and the study of the effects of this folic acid antagonist on certain folate-dependent enzymes.